Leukemoid Reaction vs CML: How to Tell Them Apart
Both leukemoid reaction and CML produce a dramatic leukocytosis with a left-shifted myeloid picture that can look alarming on a first glance at the CBC. The core axis that separates them is reactive versus neoplastic: a leukemoid reaction is a benign, exaggerated marrow response to an underlying stressor, while CML is a clonal myeloproliferative disorder driven by the BCR-ABL1 fusion gene. The LAP score, presence of basophilia, and detection of the Philadelphia chromosome resolve the two decisively.
How to tell them apart
| Feature | Leukemoid Reaction | Chronic Myeloid Leukemia (CML) |
|---|---|---|
| Etiology | Reactive process driven by severe infection, inflammation, or hemorrhage | Neoplastic clonal proliferation caused by the BCR-ABL1 fusion gene |
| WBC count | Usually below 50,000, though it can climb higher | Often markedly elevated, in the 50,000–200,000+ range |
| Leukocyte alkaline phosphatase (LAP) score | HIGH — reflecting metabolically active, normal neutrophils | LOW — reflecting the abnormal neoplastic neutrophils |
| Basophilia | Absent | Present — a hallmark of CML (often with accompanying eosinophilia) |
| Philadelphia chromosome / BCR-ABL1 | Absent | Present — diagnostic of CML |
| Peripheral smear findings | Toxic granulations, Döhle bodies, and a reactive left shift | Full spectrum of myeloid maturation with a myelocyte 'bulge' and no toxic changes |
| Clinical course | Resolves once the underlying cause is treated | Chronic disease requiring tyrosine kinase inhibitor (TKI) therapy |
The reasoning
Anchor first on context and the CBC: a patient with a documented severe infection, burn, or hemorrhage who develops a leukocytosis usually under 50,000 is far more likely to have a leukemoid reaction. Suspect CML instead when the count is very high (50,000–200,000+) without an obvious inflammatory trigger, when there is unexplained splenomegaly, or when constitutional symptoms like weight loss and night sweats are present. The smear arbitrates: toxic granulations and Döhle bodies point to a reactive process, whereas a full myeloid maturation spectrum with a myelocyte bulge plus basophilia points to CML. The LAP score is the quick tie-breaker — high in leukemoid reaction, low in CML — and detection of the Philadelphia chromosome (t(9;22)/BCR-ABL1) by FISH or RT-PCR confirms CML definitively.
Key tests
- Leukocyte alkaline phosphatase (LAP) score: HIGH in leukemoid reaction (active, normal neutrophils) versus LOW in CML (abnormal neoplastic neutrophils) — a classic bedside discriminator.
- Cytogenetics/FISH for t(9;22) and RT-PCR for the BCR-ABL1 transcript: negative in leukemoid reaction, positive/diagnostic in CML (RT-PCR also serves to monitor CML treatment response).
- Peripheral blood smear: shows toxic granulation, Döhle bodies, and reactive left shift in leukemoid reaction, versus a full myeloid maturation spectrum with a myelocyte bulge, basophilia, and eosinophilia in CML.
What they share
- Marked leukocytosis with a neutrophil-predominant, left-shifted differential
- Myeloid cells at various stages of maturation appearing in the peripheral blood
- Can both present with very high WBC counts that raise concern for malignancy on initial CBC
Pitfalls
- Assuming a very high WBC automatically means leukemia — a leukemoid reaction can reach high counts, so confirm with LAP score and cytogenetics rather than the number alone.
- Forgetting that basophilia and eosinophilia are hallmarks of CML; their absence in a reactive left shift is an easy discriminator that is often overlooked.
- Mistaking the reactive left shift and toxic changes of a leukemoid reaction for the maturing myeloid picture of CML — toxic granulations and Döhle bodies favor a reactive process, not neoplasia.
- Failing to appreciate that the LAP score runs opposite to intuition: it is HIGH in the benign reactive state and LOW in malignant CML.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.